That second picture isn't the inside of the blast even though it looks a little inside out. It's the evolution of the blast a fraction of a second later in time. Those bits sticking out the right and left sides are also sticking out the front pointing toward the camera, giving it an inside out appearance.

That first picture - and I am WILDLY speculating based on my understanding of how a nuclear bomb works - might be the initial triggering blast, which is a conventional explosive like any regular bomb. The second picture might be the first appearance of the actual nuclear explosion which moves faster and is overtaking the initial conventional blast.

Again, take that with a handful of salt. I'm just hazarding a guess.